Electrolytes are an essential member of an electrolytic cell or battery. In one arrangement, a battery or cell comprises an intermediate separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for making such cells in which the ion source electrode is a lithium compound or other material capable of intercalating lithium ions, and where an electrode separator membrane comprises a polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility.
Early Lithium Metal Cells
Early rechargeable lithium cells utilized lithium metal electrodes as the ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on separator structures or membranes which physically contained a measure of fluid electrolyte, usually in the form of a solution of a lithium compound, and which also provided a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven organic or inorganic fabric have been saturated with solutions of an inorganic lithium compound, such as LiClO.sub.4, LiPF.sub.6, or LiBF.sub.4, in an organic solvent, e.g., propylene carbonate, diethoxyethane, or dimethyl carbonate, to form such electrolyte/separator elements. The fluid electrolyte bridge thus established between the electrodes has effectively provided the necessary Li+ ion mobility at conductivities in the range of about 10.sup.-3 S/cm.
Ion, Rocking Chair Cells and Polymer Cells
Although serving well in this role of ion conductor, these separator elements unfortunately comprise sufficiently large solution-containing voids that continuous avenues may be established between the electrodes, thereby enabling lithium dendrite formation during charging cycles which eventually leads to internal cell short-circuiting. Some success has been achieved in combatting this problem through the use of lithium-ion cells in which both electrodes comprise intercalation materials, such as lithiated manganese oxide and carbon (U.S. Pat. No. 5,196,279), thereby eliminating the lithium metal which promotes the deleterious dendrite growth. Although providing efficient power sources, these lithium-ion cells do not readily attain the capacity provided by lithium metal electrodes.
Another approach to controlling the dendrite problem has been the use of continuous films or bodies of polymeric materials which provide little or no continuous free path of low viscosity fluid in which the lithium dendrite may propagate. These materials may comprise polymers, e.g., poly(alkene oxide), which are enhanced in ionic conductivity by the incorporation of a salt, typically a lithium salt such as LiClO.sub.4, LiPF.sub.6, or the like. A range of practical ionic conductivity, i.e., over about 10.sup.-5 to 10.sup.-3 S/cm, was only attainable with these polymer compositions at ambient conditions well above room temperature, however. Some improvement in the conductivity of the more popular poly(ethylene oxide) compositions has been reported to have been achieved by radiation-induced cross-linking (U.S. Pat. No. 5,009,970) or by meticulous blending with exotic ion-solvating polymer compositions (U.S. Pat. No. 5,041,346). Each of these attempts achieved limited success due to attendant expense and restricted implementation in commercial practice.
"Solid" and "Liquid" Batteries of the Prior Art
More specifically, electrolytic cells containing an anode, a cathode, and a solid, solvent-containing electrolyte incorporating an inorganic ion salt were referred to as "solid batteries". (U.S. Pat. No. 5,411,820). These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety factors. Despite their advantages, the manufacture of these solid batteries requires careful process control to minimize the formation of impurities due to decomposition of the inorganic ion salt when forming the solid electrolyte. Excessive levels of impurities inhibit battery performance and can significantly reduce charge and discharge capacity.
Specifically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable inorganic ion salt as a separate component. The inorganic matrix may be non-polymeric [e.g., .beta.-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells containing a solid electrolyte containing a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer), added to the matrix primarily in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte have been art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like. These are examples of aprotic, polar solvents.
Heretofore, the solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (usually a glyme and the organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte. In another method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte. When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
A highly favored electrolyte/separator film is prepared from a copolymer of vinylidene fluoride and hexafluoropropylene. Methods for making such films for cell electrodes and electrolyte/separator layers are described in U.S. Pat. Nos. 5,418,091; 5,460,904; and 5,456,000 assigned to Bell Communications Research, each of which is incorporated herein by reference in its entirety. A flexible polymeric film useful as an interelectrode separator or electrolyte member in electrolytic devices, such as rechargeable batteries, comprises a copolymer of vinylidene fluoride with 8 to 25% hexafluoropropylene. The film may be cast or formed as a self-supporting layer retaining about 20% to 70% of a high-boiling solvent or solvent mixture comprising such solvents as ethylene carbonate, propylene carbonate, dimethyl carbonate, and dibutyl phthalate. The film may be used in such form or after leaching of the retained solvent with a film-inert low-boiling solvent to provide a separator member into which a solution of electrolytic salt is subsequently imbibed to displace retained solvent or replace solvent previously leached from the polymeric matrix.
Electrolyte Breakdown
Regardless of which technique is used in preparing an electrolyte/separator, a recurring problem has been the presence of impurities which interfere with cell function and can reduce battery life. The source of these impurities is the partial decomposition of the inorganic ion salt formed in the polymer matrix. Partial decomposition occurs due to exposure of the inorganic ion salts to the high temperatures used, for example, in forming the polymer matrix, and/or in evaporating the volatile solvent, and/or in batteries used at elevated temperatures. These high temperatures cause the salt to break down into insoluble or less soluble salts. For example, upon decomposition of lithium hexafluorophosphate (LiPF.sub.6), the decomposition product LiF is formed; and the LiF is much less soluble in the electrolyte solvent and can precipitate out. Such insoluble or less soluble salts cannot function to transfer electrons, and hence the resulting battery is rendered less efficient.
Thus, in preparing electrolyte/separator, great care must be taken to maintain processing temperatures below the threshold level for significant salt decomposition. The need for careful monitoring of process temperatures increases manufacturing costs and at the same time results in a percentage of the electrolyte/separators produced being off specification due to unavoidable process temperature variation. Electrolyte/separator materials meeting production specifications generally contain small but tolerable levels of impurities which can nevertheless affect cell performance, particularly with respect to cumulative capacity. Cumulative capacity of a battery is defined as the summation of the capacity of the battery over each cycle (charge and discharge) in a specified cycle life.
Quite apart from the problem of decomposition is the cost of the inorganic ion salts. Simple salts such as lithium halides are less preferred in the electrolyte because they are either insoluble or unstable. More complex salts are favored because of their greater compatibility, but are more costly. A highly preferred, complex salt is LiPF.sub.6, but this salt has been found to be very heat sensitive and quite expensive. Another preferred, complex salt is lithium hexafluoroarsenate. This salt poses significant disposal problems due to the presence of arsenic. Notwithstanding their complexity and costs, even under the best of circumstances (e.g. impurity levels approaching zero), the inorganic ion salts typically have a transference number between 0.4 and 0.55, meaning that the ion salt carries only between 40% and 55% of the total plus (+) charge.
In view of the above, it can be seen that it is desirable to have a novel salt which is economical, stable, does not degrade capacity; and which is usable in a variety of electrolyte/separator configurations, as described above as exemplary.